package_rpm: support signing of rpm packages

package_rpm: support signing of rpm packages

This patch adds a new bbclass for generating rpm packages that are
signed with a user defined key. The packages are signed as part of the
"package_write_rpm" task.

In order to enable the feature you need to
1. 'INHERIT += " sign_rpm"' in bitbake config (e.g. local or
   distro)
2. Create a file that contains the passphrase to your gpg secret key
3. 'RPM_GPG_PASSPHRASE_FILE = "<path_to_file>" in bitbake config,
   pointing to the passphrase file created in 2.
4. Define GPG key name to use by either defining
   'RPM_GPG_NAME = "<key_id>" in bitbake config OR by defining
   %_gpg_name <key_id> in your ~/.oerpmmacros file
5. 'RPM_GPG_PUBKEY = "<path_to_pubkey>" in bitbake config pointing to
   the public key (in "armor" format)

The user may optionally define "GPG_BIN" variable in the bitbake
configuration in order to specify a specific gpg binary/wrapper to use.

The sign_rpm.bbclass implements a simple scenario of locally signing the
packages. It could be replaced by a more advanced class that would
utilize a separate signing server for signing the packages, for example.

[YOCTO #8134]

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>